The Preview PDF tab appears only when Acrobat or Adobe Reader is installed. By default, if you have both Acrobat and Adobe Reader installed, LiveCycle Designer ES starts Acrobat automatically to preview the form. To use Adobe Reader to preview the form, you must start it before you click the Preview PDF tab.
For example, if you select Acrobat and Adobe Reader 6.0.2 or later as the target version for the form, ensure that you have Acrobat and Adobe Reader 6.0.2 installed.
The preview will use the same format as the saved form. You can right-click the PDF Preview tab to see which default file type option is currently selected. To indicate the format of an unsaved form, you must change the Tools > Options > Document Handling setting.
If you are designing forms for use with LiveCycle Forms ES, the same form design can be used to render PDF or HTML forms. Although you can preview the form design on the Preview PDF tab, the HTML form may not appear the same as it does when the form is rendered by using LiveCycle Forms ES. If you are creating an HTML form, render the form by using LiveCycle Forms ES and preview it in a web browser.
See also 

About previewing and testing forms